1. Field of the Invention
The invention is directed to non-impact printer apparatus and methods of controlling same and particularly to those non-impact printers employing a plurality of recording elements emitting light for recording upon a photosensitive recording medium.
2. Description of the Prior Art
In the prior art as exemplified by U.S. Pat. No. 3,850,517, light-emitting diode (LED) arrays for use in printing are described. Specifically, printing may be accomplished by selectively turning selected LED's on or off to modulate the charge upon a photosensitive recording medium such as photographic film or upon an electrostatically charged photoconductor. In the case of the photoconductor, the modulation of the electrostatic charge may be developed with appropriate charged toners and a permanent record made by transferring and fusing the toned image to a receiver sheet such as plain paper, for example. In order to provide image quality that is satisfactory, it is required that the amount of light used to create each pixel (picture element) of image information be controlled so that exposure upon the recording medium will not be over or underexposed. To this end, the aforementioned patent provides a prism with a linear photocell in the exposure path. A portion of the light reaches the photocells and another portion impinges upon the recording medium. The LED's are driven sequentially; i.e., during a period for exposing one line of pixels, i.e., one is on and then turned off and then the next LED turns on. The electronic controls determine when enough light has impinged upon the photocell, and hence the recording medium, and turns the LED off and the next LED to be illuminated is turned on and so on. A problem with this type of printer is that it would be desirable to record a line of pixels simultaneously. In order to do so with the described printer, such would require a timing circuit for each LED where thousands of LED's on a printhead are typical.
U.S. Pat. No. 4,780,731, the contents of which are incorporated herein by this reference, describes an LED printhead where control over LED activation times is made electronically. Photoelements are moved into position either by traversing one across the printhead from LED to LED or by swinging an entire assembly of photoelements into position. The photoelements measure LED light output during periods when printing is not occurring and determine the need for adjustment of LED activation times. As noted in this patent, the required activation time for each LED changes during the life of the LED printhead; i.e., aging effect, and also changes dependent upon the operating temperature of the printhead.
A problem with this prior art printer apparatus is that in measuring the light output of the LED's, measurements are not made at the focal plane of the lens. Because of this, one is required either to make the measurements for each LED one at a time to avoid cross-talk or to remove the recording medium and move the photosensor into position at the focal plane. The latter is very undesirable, particularly when corrective measurements are to be made frequently during the life of the printhead.
The objects of the invention are therefore to overcome the disadvantages noted in the prior art.